I. Field of the Invention
This invention relates generally to control of molding machinery. In particular this invention relates to servo control of injection molding machine devices.
II. Description of the Prior Art
It is known to provide servo control for injection molding machine mechanisms including. Or example, a clamp for operating mating die components defining a mold cavity, an extruder for plasticizing material to be formed into a molded article an injection ram for effecting injection of the plasticized material into the mold cavity, and an ejector for removing the finished article from the mold cavity. In machines wherein the actuators are hydraulically operated, such servomechanism control is effected through electrically operated hydraulic valves controlling hydraulic cylinders and motors. It is also known to provide servo control of injection molding machine heaters to control the temperature of the material to be injection molded
From U.S. Pat. No. 4.745.541 it is known to provide digital devices to effect servo control of molding machine mechanisms wherein the control algorithms are implemented by execution of programs by, for example, a microprocessor. The control algorithms are defined as mathematical relationships of measured parameters and control signals, the relationships including proportional, integral and derivative (PID) components. It is known from this reference to perform algorithm processing upon occurrence of a timed interrupt and to select an algorithm to be processed from a plurality of active control algorithms, While overcoming the inherent disadvantages of analogue control implementations, the control of this reference suffered the disadvantage that only a single loop closure interval was provided by the analogue signal processor. The control processor of this reference was not, therefore suitable for performing servo control of both machine mechanisms and heaters. Further, the servo control of this reference did not provide for control of a single analogue output in response to more than a single PID algorithm at any time. Consequently, the control signal was not effective to perform for example hydraulic pressure relief while implementing velocity control.